Gelatin is commonly used as a binder for various photographic materials. Quite often, several layers in a photographic element, such as light-sensitive silver halide layers, overcoat layers, filter layers, interlayers, antihalation layers, backing layers, film base subbing layers, antihalation layers and baryta layers contain gelatin as a major component.
Photographic materials are generally processed in aqueous processing solutions. Such processing results in excessive swelling and loss of strength of the gelatin binder unless the gelatin has been hardened. Also, if the processing solution is maintained at a warm temperature, such as 40.degree. C., the gelatin binder can dissolve, causing the layers of the element to disintegrate, unless the gelatin has been hardened.
A number of compounds have been used to harden gelatin. Typical examples of hardeners include formaldehyde and aldehyde compounds as described in U.S. Pat. No. 3,232,764, active esters as described in U.S. Pat. No. 3,542,558, compounds that contain a reactive halogen atom as described in U.S. Pat. No. 3,951,940, activated olefins as described U.S. Pat. No. 3,642,486, aziridine compounds as described in U.S. Pat. No. 3,017,280, epoxy compounds as described in 3,091,537, inorganic hardening agents such as chromium alum and zirconium sulfate and others well-known in the art. These compounds harden or crosslink gelatin, thus increasing its mechanical strength and reducing the swellability and solubility of the gelatin in aqueous processing solutions.
The above-described hardening compounds are, however, subject to a number of disadvantages. In some cases an exceedingly long time is required after a layer is cast and dried to achieve the desired degree of gelatin hardening; thereby requiring photographic elements containing gelatin hardened with those compounds to be aged for an extended time while the hardening process is completed. In other words, these hardeners exhibit after hardening. Additionally, a number of compounds exhibit adverse effects on the photographic properties of elements in which they are used. Such adverse effects can include an increase in fog or a reduction in light sensitivity or photographic speed.
In order to avoid the time and expense of keeping large quantities of gelatin-containing photographic elements for long periods while the hardening process is completed, a number of socalled fast-acting hardeners have been described in the art. For example, U.S. Pat. No. 4,063,952 describes a carbamoylpyridinium salt hardening compound, European Patent Application 162,308 describes a chloroformamidinium hardener and U.S. Pat. No. 4,612,280 describes an N-succinimidyloxyformamidinium hardener. These hardeners, however, suffer from a number of problems. Some of these hardeners exhibit after hardening. Others have severe handling difficulties due to their hygroscopic nature and/or to poor hydrolytic stability. Also, some of these hardeners adversely affect either the physical properties of photographic elements in which they are used (e.g., tackiness) or their sensitometric properties (e.g., speed loss). These problems are often aggravated by the fact that relatively large amounts of hardener are often required to achieve the desired hardening effect, leading to relatively large amounts of sometimes deleterious by-products.
It is therefore desirable to provide a hardener that effectively hardens gelatin in a short period of time, while substantially avoiding or reducing many of the above-described adverse effects. It is toward that end that the present invention is directed.
In the photographic arts, there is an abiding interest in the provision of new classes of hardener compounds such as provided herein. In view of this interest and in view of the hardening properties of the compounds of this invention, it is believed that this invention represents a substantial contribution to the art.
Ballantine et aI., U.S. Pat. No. 3,951,940 describes N-alkyl-2-halopyridinium salts and alkylene-N,N'-[bis(2-halopyridinium)] salts for hardening gelatin, shown in formulas (II) and (III). ##STR2## These compounds have a 2-pyridylium moiety bonded to a halogen atom. They do not suggest the compounds of this invention, in which a 2-pyridylium moiety is bonded to a nitrogen which carries a positive charge. The Ballantine et al. compounds contain a halogen on the pyridinium ring that is released upon reaction with gelatin, which may cause adverse photographic effects in photographic elements that contain silver halide. Such halogen need not be present in the compositions of this invention. Furthermore, when halogen is present as a substituent on the 2-pyridylium moiety in compositions of this invention, the halogen is deliberately not conjugated with the quarternary ring nitrogen and is therefore not released during reaction of these compounds with gelatin.
Chen et al., U.S. Pat. No. 4,877,724, discloses dicationic ethers as useful hardeners. The hardeners of this invention are decidedly different from those within Chen et al., since the compounds of this invention do not have the ether moiety that is within the hardeners of Chen et al..